I don't see any on Ebay right now but have in the past. They have a couple of complete Nutone 251 units for sale but no control knobs. I would keep checking back with them.
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Michael and Georgia Day
1992 Newell 43.5' #281
8V92 DDEC-2, HT740
PT Cruiser GT with Remco Transmission Pump https://newellowner.com/newell-photos/